Field of the Invention
The present invention relates to a bowing measurement method and apparatus for measuring the amount of bowing of a silicon substrate, a sapphire substrate, and other single-crystal substrates.
Description of the Related Art
In recent years, attention has been directed toward GaN (gallium nitride), AlN (aluminum nitride), SiC (silicon carbide), and other wide-bandgap semiconductor materials. For example, the following semiconductor parts have been receiving attention:
(1) A part used in an LED (light emitting diode) and formed of a single-crystal structure made of GaN deposited on a single-crystal sapphire substrate (GaN/sapphire);
(2) A part used in a power device and formed of a single-crystal structure made of GaN deposited on a single-crystal silicon (Si) substrate (GaN/Si);
(3) A part used in a frequency filler that is an SAW (surface acoustic wave) device and formed of a single-crystal structure made of AlN deposited on a single-crystal Si substrate (AlN/Si); and
(4) A part used in a power device and formed of a single-crystal structure made of SiC deposited on an appropriate single-crystal substrate.
A single-crystal structure made of a semiconductor material or any other material deposited on a single-crystal substrate as described above desirably has a precisely flat surface. In practice, however, a crystal growth process or an epitaxial film formation process causes a single-crystal substrate to have bowing in some cases. When the single-crystal substrate has bowing, the bowing may affect device characteristics of a final product and cause a problem in using a process technology for manufacturing a final product (that is, technology for forming a variety of elements on a substrate).
Since whether or not a single-crystal substrate has bowing greatly affects characteristics of a final product manufactured based on the single-crystal substrate, it is very important to evaluate the amount of bowing.
A laser-beam-based method has been known as a method for measuring the amount of bowing of a single-crystal substrate (Non-Patent Citation 1, for example). The method includes irradiating a specimen surface with a plurality of collimated laser beams, measuring the position of each of the laser beams reflected off the specimen surface, and evaluating the amount of bowing of the specimen based on the distribution of the positions. In the conventional laser-beam-based method for measuring the amount of bowing, however, the exterior appearance of a target object is observed, but the amount of bowing of a crystal lattice plane of a single-crystal substrate is not measured.
As a method that allows measurement of the amount of bowing itself of a crystal plane, Patent Citations 1 and 2, for example, disclose conventional X-ray-based measurement methods. The conventional measurement methods include the steps of irradiating a surface of a target object with X-rays in a position under measurement to acquire a rocking curve and determining a peak position of the rocking curve. The methods further include moving the X-rays and the target object relative to each other and carrying out the same steps on one or more positions under measurement to determine a peak position of a rocking curve in each of the positions under measurement. The methods finally include calculating the curvature radius (that is, the amount of bowing) of the single-crystal substrate based on how the peak position of the rocking curve changes when the position under measurement is changed.
FIG. 11A shows an example of the relationship between the bowing and the curvature radius. Specifically, the amount of bowing is the distance C from the line that connects both ends of a measured area A of a single-crystal substrate 101. The horizontal axes of FIGS. 11B and 11C represent the curvature radius R corresponding to the bowing C. In FIGS. 11B and 11C, the vertical axes represent the amount of bowing C, and the horizontal axes represent the curvature radius R.